UK light-heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight 10-round contest at the Manchester venue.
But Parker had his moments, delivering effective individual punches in a contest where the techniques didn't complement each other, with frequent clinching marring the contest.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They succeed," Buatsi said in the arena as he was met by disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the decision but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
The Champion Secures Victory in Uneventful Match
His career development has stalled in recent years through management issues and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors prepared to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who operated with a tight guard to edge the cleaner rounds.
The audience's tolerance decreased during frequent holding, with the challenger slipping to the canvas various instances during clinches.
Parker ended stronger, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor advances, but with the presence of power puncher Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester battle to capture the European light-heavyweight championship.
The stadium fell silent for a deeply poignant homage ahead of the contest as 10 bells tolled in remembrance of the city's native Ricky Hatton, who died aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound disappeared, the silence became a shout of "there's just one Ricky Hatton".
The action during the bout matched the sentiment. The eventual winner was knocked down by a powerful punch flush on the chin in the second round, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was injured soon, heavy swelling developing under his orbital area as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the action seldom decreased over twelve rounds.
The victor edged the verdict with scoring tallies of even score, narrow lead and clearer margin, but each competitor were relentless and full of heart, exactly what their inspiration Hatton embodied.
